The invention relates to a belt conveyor with active part of the belt supported by a trough wall through the medium of a gas layer. Such a belt conveyor is known from U.S. Pat. No. 5,889,802.
The belt conveyor is one of the most widely used means of conveyance for bulk goods. This is partly due to its relatively low energy consumption. Other conveyors, such as e.g. vibrating or screw conveyors, use 10 to 100 times the energy per ton km. For plants with great conveyance distances and high conveyance capacities in particular, low energy consumption per ton km of conveyed bulk goods is extremely important from the point of view of costs. In the designing of such a plant, every effort will therefore be made to achieve the lowest energy consumption.
In the hitherto most commonly used design of belt conveyors, the belt is supported by supporting rollers. The energy consumption per ton km of conveyed material is determined here by the resistance forces which the belt encounters during conveyance. In a new plant, all rollers will run smoothly and the belt resistance--which is the sum of all resistance forces--will be low. However, as time passes, wear and soiling of the bearings will cause an increasing number of rollers to drag more, which increases the belt resistance. An inadmissibly great belt resistance can be avoided only by replacing dragging rollers in good time by new or repaired rollers. The costs involved here do, however, constitute a considerable item in the overall running costs of the belt, and an optimum will have to be found between not too frequent maintenance work, on the one hand, and not too high energy consumption through dragging rollers, on the other.
Another design of belt conveyor is the above-mentioned air belt conveyor. Here, the belt is supported over its entire length by a trough. By blowing air under the belt via openings in the trough, by means of a compressor plant, e.g. a centrifugal compressor, a thin film of air is produced between the belt and the trough, as a result of which the belt resistance is much lower than it is in the absence of the air film.
Unlike the belt conveyor with supporting rollers, in the case of the air belt conveyor the energy consumption is not solely determined by the energy required for driving the belt, but also by the energy needed to drive the compressor device (for the sake of brevity, called a compressor below). The total energy consumption is therefore the sum of the energies for the driving of the belt and for the driving of the compressor. In the design of an air belt conveyor, the air flow capacity of the compressor will also have to be determined. A large capacity, thus a large output of air flow, means a great thickness of air film and thus low belt resistance. Although the energy required for driving of the belt is low then, a large amount of energy is needed for driving the compressor, and the total energy consumption will be high as a result, i.e. high in relation to the energy consumption of a belt conveyor with supporting rollers. Likewise, a compressor with small capacity will require little energy for driving it, but owing to the low air film thickness, the belt resistance will be high, and this will make the total energy consumption high. The job of the designer of the conveyor is to select a compressor with the right capacity, so that the total energy consumption is low.
One difficulty here is that the conditions in which the conveyor operates are not constant. The optimum capacity of the air pump as regards total energy consumption is determined by a number of factors. One important factor is e.g. the loading of the belt. In a conveyor which generally operates with a fully loaded belt, the optimum compressor will be quite different from that of a conveyor generally operating with a half-loaded belt. Another factor which has an effect is whether there is continuous uniform loading of the belt, or loading which varies widely or is often interrupted, so that parts of the belt are loaded while others are not. The quality of the trough also plays an important role. If the trough is e.g. badly manufactured and/or mounted, so that the surface of the trough has many uneven zones which cause direct local contact between the belt and trough, thereby causing great belt resistance, a thicker film of air, and thus a compressor of larger capacity, will be needed than in the case of a smooth trough with few uneven zones, to decrease this resistance.
Through use, the uneven zones in the badly manufactured trough will, however, in the course of time gradually wear off by the belt, continuously rubbing thereon, so that the conveyor becomes increasingly smoother and the belt resistance decreases. The value of the optimum compressor capacity will also be reduced as a result in the course of time.
The fact that in the case of the air belt conveyor energy has to be fed in at two points therefore, on the one hand, gives the designer possibilities--through the right selection of the ratio of the two energy supplies--of achieving the optimum situation with a minimum total energy consumption. On the other hand, making the right choice is very difficult and, owing to the changing influence factors, the actual situation as regards energy consumption will generally deviate from the optimum situation.